1. Field of the Invention
This invention relates to an electronic system for process instrumentation and control or more in particular to a power supply system with a high reliability.
2. Description of the Prior Art
In this specification, the electronic system for process instrumentation and control is indicative of a system whereby process variables are measured or controlled electronically.
A process variable such as a flow rate is converted into a DC current signal of 4 to 20 mA for transmission. This current signal is applied to an input converter where it is converted into a DC voltage signal of 1 to 5 V suitable for an input to a measuring instrument connected in a subsequent stage. In the case where a process variable is a flow rate and a differential pressure transmitter is used as a detector, a rooter amplifier is employed as the instrument arranged in a stage subsequent to the input converter. In response to the output of the input converter, the rooter amplifier produces a voltage signal proportional to the process variable. A rooter amplifier, as defined in the "Electronics and Nucleonics Dictionary" published by the McGraw-Hill Publishing Company is a non-linear amplifier in which negative feedback is used to make the output voltage vary as the square root or some other root of the input voltage. The voltage signal, having been processed by the rooter amplifier, is applied to a PID controller. The PID controller determines a difference (a deviation) between an input signal thereto and a set signal and produces the result of the PID (proportional plus integral plus derivative) operation. The output of BID controller is converted into a DC actuation signal of 4 to 20 mA by an output converter, thus causing an actuator to open and close a valve. For the purpose of integration, the signal from the rooter amplifier is applied to an integrator in parallel to the PID controller, so that the result of integrating operation effected in the integrator is indicated by a counter or the like.
Power is supplied to the input and output converters and operational elements or operators from a stabilized DC power source which rectifies and stabilizes a commercial power source to provide a DC source voltage of 24 V. The reason for supplying power not directly from a commercial power source but through the stabilized DC power source is to eliminate ill effects due to possible frequency and voltage fluctuation of the power source, to suppress noise from the power source and to facilitate a power back-up during power failure.
On the other hand, there are a number of methods for supplying DC power to the units of the electronic system for process instrumentation and control including, in the case under consideration, the rooter amplifier, PID controller, integrator and output converter. One method is the direct power supply method in which the DC power supply is used directly as a power source for the circuits. The other method is the insulated power supply method in which DC power is subjected to DC-DC conversion and is then fed to power supply terminals of each unit through a transformer.
The disadvantage of the direct power supply method is that a trouble in an instrumentation and control system has an adverse effect on another instrumentation and control system through the power line due to direct connection of a plurality of instrumentation and control systems to the same power supply. For example, in the event that an abnormally high voltage makes its way into a first instrumentation and control system through connection of the transmitter or actuator therewith, by induction or contact with a high voltage, the circuit elements in each unit of the first instrumentation and control system may be damaged, or continuous application of abnormally high voltages results in the same effect as of successive occurrence of noise. This gives rise to erroneous operation, deteriorating the control characteristics. Not only that, the abnormal voltage is undesirably led to a second instrumentation and control system through voltage lines, thus damaging or deteriorating the control characteristics of the circuit elements in each unit of the second instrumentation and control system. Also, if the power supply is short-circuited in a given unit of the first instrumentation and control system, an excess current protection circuit in the stabilized DC power source is actuated, with the result that power to the second instrumentation and control system is cut off, thus rendering the same inoperative.
A method often employed to prevent this inconvenience is to insert a fuse in a power lead-in terminal of each unit, which fuse is adapted to burn out in response to the short-circuiting in the power supply. This method, however, greatly undermines the safety of the objects of control. In ordinary plant operation the direction in which the actuator is closed or opened is selected in such a manner as to orient the plant operation in a safe direction at the time of power failure. In other words, the actuator is operated in a safe direction when the signal to the actuator, namely, the output signal of the output converter is zero. This output signal in a safe direction is incapable of being produced in response to the burning out of any of the fuses inserted in the power circuits of the respective units of an instrumentation and control system. In the aforementioned case involving a flow rate as a process variable, for example, the burning out of the fuse of the output converter renders the output thereof zero, thus directing the plant operation in a safe mode. In the case where one of the fuses inserted in the power circuits of the input converter, the rooter amplifier and the PID controller burns out with the output converter operative, however, it is impossible to orient the output of the output converter in the safe direction.
This shortcoming of the direct supply method is even more aggravated from the viewpoint of safety of the plant operation when the number of instrumentation and control systems deriving power from a single stabilized DC power supply reaches as large as several tens of loops.
In the insulated power supply method in which a DC-DC converter with a transformer is provided at the power supply terminals of each unit and an insulated power supply is used as a circuit power source, by contrast, the effect of a trouble of one instrumentation and control system on another accompanying the above-mentioned direct power supply method is eliminated by the transformer of the DC-DC converter. In spite of this, the disadvantage remains unresolved that the aforementioned actuator terminal cannot be oriented in the safe direction in the event of a power failure caused by the burning out of the fuse in such units as the input converter and the PID controller or a fault of the DC-DC converter. Further, the fact that each unit must be provided with a DC-DC converter leads to a complicated circuit configuration as well as to a high cost.